Decouping 101 by electronic peasant
Circuits that are wired together are coupled. Sometimes there are connections
that you don't want coupled. You don't want the lights to dim everytime you open your
garage door. So you decouple them.
Decoupling them usually involves using capacitors as local power storage devices.
Wires have resistance and inductance. Resistance causes a permanent voltage drop
(usually small)... Inductance causes the voltage to drop when the current demand
changes... when you 'suck' current, it drops... when you 'stop sucking' it
overshoots. The fix is to put a large enough capacitor RIGHT next to the device that will
'suck' current... so that when it starts to suck it gets the first mouthful of electrons
from the capacitor... and satisfies its thirst while the inductance of the power
supply wiring is overcome. Eventually... the power will get there. When the current flow
stops... the remaining current in the power supply wiring will keep on flowing to the
last place it was headed... and will dump into the cap instead of the device. The cap
should be big enough to supply the sudden demand, and take up the additional charge
when the load shuts off.
This DOES NOT happen at the speed of light... it is much slower... often 60% or
less than light speed. Thats why decoupling is necessary. Decoupling assures that
individual chips function properly... and that unwanted interaction through the
supplies does not occur. Often cited is the 555 timer... this is one current HOG every
time the output changes state... it draws a huge current pulse from the supply. In this
case its necessary to have more than one decoupling cap... usually a large, slow
electrolytic or faster tantalum for the bulk of the charge... in parallel with a small
ceramic cap. The two in parallel have a synergistic effect... the smaller capacitor
(with usually very low series inductance) can supply a small current very fast. The
larger cap (with higher inductance) takes longer to get into action. The reverse
occurs when the current draw of the load stops.
These spikes... if allowed to rock the power supplies, can cause audible noise...
unintended VCO sync, things like that. Good decoupling is essential for the 78xx and
79xx series regulators. Follow the data sheets and use capacitors located right at
(as close as POSSIBLE) the regulator pins... even if the main supply cap is right
nearby... big caps have big inductance... they are slow.
Another form of decoupling is sometimes used for unused pins... a good example is pin 5
(control voltage in) of the 555 timer. When it is not used... it can pick up unwanted
signals (noise) and cause the frequency or period of the 555 to go off. Data sheet
recommends a .01 or .1uF cap to ground if you are not using this pin.
How much decoupling do I use ??? LOTS. With op-amps I place them between every
package... for CMOS, at least every other package... for a 555, two caps in
parallel... and I run the power supply wiring right back to the main source (power
input to the card) separately.
There is one other form of decoupling... using a resistor in series with the power
supply leads... and a much larger cap. This forms a low-pass filter that eliminates most
low frequency noise from sensitive circuits (usually things like preamps or some filter
stages). You choose the resistor to be as large as you can...allowing that the voltage
to that chip is going to drop... maybe on a +/-12V circuit you could run that preamp at
+/-10V... then size the resistor to drop 2V. Of course the more you drop... the less
output power you will be able to deliver...Then you make the capacitors as
large as you need to... to get rid of the unwanted ripple. Thats one good reason to
use full wave DC supplies (and not halfwave) but that can be next weeks class, eh?
Again... if you use big caps... bypass them with some little ceramic caps...If your
hobby takes you up to radio frequencies... sometimes three caps in parallel are used,
including surface mount types... to get rid of the last little bit of lead inductance.
BTW: inductance is minimized by keeping the path that the current flows in SMALL, which
is why you locate decoupling caps as near as possible to the load. When I do a layout...
decoupling caps go in first... before all other components